1) Weight savings, even though the rear seats REALLY do not weigh much. Maybe 10-20 pounds. I've removed mine when I reupholstered the interior of my V6, and I was surprised how light they were.
2) Because they're useless unless children sit in the back. No human adult will be comfortable back there unless it's at the expense of the people in the front seats.
3) Because race car.
1. makes car sound better on the inside (stereo/muffler wise)
2.gives me a good excuse to not put people back there
3. more room for activities
4. because race car.